INSTINCT 109 
ings of many instinctive acts. Hunger drives the lioness 
to seek for prey; sexual impulses lead to the search for mates; 
and a bird in confinement may become uneasy when the 
time, for migration arrives. The same thing is even more 
conspicuously illustrated in the instinct of play. A lamb 
may frisk about from sheer good feeling. A kitten may 
crouch and spring as if upon a mouse when there is no ex- 
ternal object to excite its action. And where the play 
activities are associated with external objects, the latter 
serve only to awaken the stored energy of impulse which 
may be nearly ready to discharge on its own account. The 
play impulse which may sometimes vent itself in random 
movements usually takes fairly definite channels of expres- 
sion which are quite characteristic of particular species of 
animals. The energies of the young animal tend to dis- 
charge themselves in movements similar to those which form 
the regular behavior of the adult, and a certain degree of 
proficiency is reached in those activities which form the 
more serious occupations of later life. But the promptings 
to such behavior are due mainly or wholly to internal 
impulses. 
The element of internally initiated impulse in instinct is 
not confined to higher forms. It is probably coextensive 
with animal life. Amid all the stereotyped responses of 
the Protozoa we have a large element of activity determined 
by internal factors. The almost constant swimming of 
many infusorians, and the regular rhythmical activity of 
others are, like the beating of the heart and other organic 
rhythms, the result of causes within the organism. 
It is of course difficult in many cases to ascertain whether 
activity results, perhaps indirectly, from outer stimulations 
or from internal changes. In a great many cases the 
organism needs but a slight provocation to discharge its 
